The idea of applying a nitride semiconductor to a high-voltage, high-power semiconductor device by taking advantage of features such as a high saturated electron velocity and a wide band gap is under consideration. For example, GaN that is a nitride semiconductor has a band gap of 3.4 eV larger than the band gap (1.1 eV) of Si and the band gap (1.4 eV) of GaAs and has a high breakdown electric field strength. For this reason, GaN is a very promising material for a power semiconductor device for high-voltage operation and high power.
Semiconductor devices using a nitride semiconductor include field-effect transistors. There have been numerous reports on field-effect transistors, particularly high electron mobility transistors (HEMTs). For example, among GaN-based HEMTs (GaN HEMTs), an AlGaN/GaN HEMT using GaN in an electron transit layer and AlGaN in an electron supply layer is attracting attention. In an AlGaN/GaN HEMT, distortion occurs in AlGaN due to the difference in lattice constants between GaN and AlGaN. Piezoelectric polarization resulting from the distortion and spontaneous polarization of AlGaN lead to formation of a high concentration of two-dimensional electron gas (2DEG). Accordingly, an AlGaN/GaN HEMT is expected to serve as a high-efficiency switching device or a high-voltage power device for an electric vehicle or the like.    [Non Patent Document 1] D. Song et al., IEEE Electron Device Lett., Vol. 28, No. 3, pp. 189-191, 2007
An electrode of a semiconductor device using a nitride semiconductor (e.g., a gate electrode of a GaN-HEMT) uses a laminated structure of nickel (Ni) and gold (Au). Ni is a metal having a relatively high melting point, and a favorable Schottky barrier is formed between Ni and GaN. Ni also has high resistivity. The structure in which Au having low resistivity is deposited on Ni lowers the resistance of the gate electrode and prevents deterioration in high-frequency characteristics. Generally, a gate electrode has a so-called overhang structure to reduce electric field concentration on its edge, i.e., plugs a through-hole formed in an insulating film (passivation film) which protects a surface of a nitride semiconductor with a gate material and overhangs the passivation film. In this case, a film of Ni formed on monocrystals of the nitride semiconductor is affected by the crystal structure of the underlying layer and has a face-centered cubic structure (fcc) (111) orientation. A film of Ni formed on the passivation film is affected by the amorphous structure of the underlying layer and has a random orientation. For this reason, a very large grain boundary is formed between the Ni having the fcc (111) orientation and the Ni having a random orientation.
Additionally, an interface (sidewall interface) between Ni and a sidewall of the through-hole in the passivation film is likely to split open due to thermal stress or the like.
If the GaN-HEMT with the above-described structure is electrified at a high temperature, Au of the gate electrode passes through a large grain boundary and a sidewall interface and reaches a nitride semiconductor surface (Schottky surface) to react with the nitride semiconductor surface. This deteriorates gate characteristics and lowers the reliability of the GaN-HEMT.